1. Field of the Invention
This invention relates to heat sinks and, more specifically, to heat sinks wherein heat is absorbed by the phase change of a phase change material.
2. Brief Description of the Prior Art
For certain applications, electronic circuit board and component heat sinks are built with embedded phase change material (PCM). Phase change materials for such purposes are well known in the art, an example thereof being a wax which preferably has a unitary melting temperature, paraffins of this type being readily available and well known. The PCM absorbs waste heat as it changes from the solid state to the liquid state. PCMs are also available which can further change from the liquid state to the gaseous state or merely operate in the latter two phase states. Currently, heat sinks which use a PCM are built in several ways. One way is to machine thermally conductive fins in a thermally conductive plate, such as, for example, aluminum or copper. PCM is poured into a cavity containing the fins and a lid is used to seal the PCM volume. Heat is drawn to the fins and then from the fins into the PCM with the PCM changing phase as it absorbs the heat. An alternate means of construction is to build a similar assembly using commercially available thermally conductive fin stock. In this alternate configuration, the fin stock is vacuum brazed or dip brazed to a thermally conductive plate.
In both cases, the thermal efficiency of the system is limited by the minimum obtainable feature sizes of machined fins and fin stock since the amount of heat transferred from the fins to the PCM is related to the amount of fin surface contacting the PCM as well as the amount of PCM material available for phase change. The goal is to have the minimum fin thickness and the minimum distance between fins while having the maximum possible volume of PCM in the cavity area. Maximum PCM volume is obtained by having the minimum fin thickness. The minimum distance between fins is required to reduce the thermal resistance between the fin and the PCM melt front. With either machined fins or fin stock, heat from localized sources is transferred directly into the PCM under the heat source and along the fin length to the PCM not located directly under the heat source. Fins essentially provide two dimensional heat flow. This limits the thermal performance of the heat sink.